Flexography is a method of printing that is commonly used for high-volume runs. Flexographic printing plates are employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography.
Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made. A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing or support layer; one or more unexposed photocurable layers; optionally a protective layer or slip film; and often, a protective cover sheet.
The support (or backing) layer lends support to the plate. The support layer can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefins, polyamides, and the like. One widely used support layer is a flexible film of polyethylene terephthalate.
The photocurable layer(s) can include any of the known polymers, monomers, initiators, reactive and/or non-reactive diluents, fillers, and dyes. As used herein, the term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional relief pattern of cured material. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety.
Photocurable materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, “actinic radiation” refers to radiation that is capable of polymerizing, crosslinking or curing the photocurable layer. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the ultraviolet (UV) and violet wavelength regions.
The slip film is a thin layer, which protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light, and the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate.
In a “digital” or “direct to plate” process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer, which is generally a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are then ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Examples of laser ablatable layers are disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety.
Processing steps for forming flexographic printing plates with relief image printing elements typically include the following:                1) Image generation, which may be mask ablation for digital “computer to plate” printing plates or negative production for conventional analog plates;        2) Back exposure to create a floor layer in the photocurable layer and establish the depth of relief;        3) Face exposure through the mask (or negative) to selectively crosslink and cure portions of the photocurable layer not covered by the mask, thereby creating the relief image;        4) Development to remove unexposed photopolymer by solvent (including water) or thermal development; and        5) If necessary, post exposure and detackification.        
Removable coversheets are also preferably provided to protect the photocurable printing element from damage during transport and handling. Prior to processing the printing elements, the coversheet is removed and the photosensitive surface is exposed to actinic radiation in an imagewise fashion. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable developer solvent (or alternatively, thermal development) removes the unexposed areas of the photopolymerizable layer, leaving behind a printing relief that can be used for flexographic printing.
As used herein “back exposure” refers to a blanket exposure to actinic radiation of the photopolymerizable layer on the side opposite that which does, or ultimately will, bear the relief. This step is typically accomplished through a transparent support layer and is used to create a shallow layer of photocured material, i.e., the “floor,” on the support side of the photocurable layer. The purpose of the floor is generally to sensitize the photocurable layer and to establish the depth of relief.
Following the brief back exposure step (i.e., brief as compared to the imagewise exposure step which follows), an imagewise exposure is performed utilizing a digitally-imaged mask or a photographic negative mask, which is in contact with the photocurable layer and through which actinic radiation is directed.
After imaging, the photosensitive printing element is developed to remove the unpolymerized portions of the layer of photocurable material and reveal the crosslinked relief image in the cured photosensitive printing element. Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include the use of an air knife or thermal development, which typically uses heat plus a blotting material. The resulting surface has a relief pattern, which typically comprises a plurality of dots that reproduces the image to be printed. After the relief image is developed, the resulting relief image printing element may be mounted on a press and printing commenced. In addition, if necessary, after the development step, the relief image printing element may be post exposed and/or detackified as is generally well known in the art.
The shape of the dots and the depth of the relief, among other factors, affect the quality of the printed image. It is also very difficult to print small graphic elements such as fine dots, lines and text using flexographic printing plates.
In addition, maintaining small dots on flexographic plates can be very difficult due to the nature of the platemaking process. In digital platemaking processes that use a UV-opaque mask layer, the combination of the mask and UV exposure produces relief dots that have a generally conical shape. The smallest of these dots are prone to being removed during processing, which means no ink is transferred to these areas during printing (i.e., the dot is not “held” on plate and/or on press). Alternatively, if the dots survive processing they are susceptible to damage on press. For example small dots can fold over and/or partially break off during printing, causing either excess ink or no ink to be transferred.
As described in U.S. Pat. No. 8,158,331 to Recchia and U.S. Pat. Pub. No. 2011/0079158 to Recchia et al., the subject matter of each of which is herein incorporated by reference in its entirety, a particular set of geometric characteristics can define a flexographic printing plate dot shape that yields superior printing performance, including but not limited to (1) planarity of the dot surface; (2) shoulder angle of the dot; (3) depth of relief between the dots; and (4) sharpness of the edge at the point where the dot top transitions to the dot shoulder.
Flexographic printing elements can additionally be made from liquid photopolymer resins and have the advantage that the uncured resin can be reclaimed from the non-image areas of the printing elements and used to make additional printing plates. Liquid photopolymer resins have a further advantage as compared to sheet polymer in terms of flexibility, which enables the production of any required plate gauge simply by changing the machine settings. The plates are typically formed by placing a layer of liquid photopolymerizable resin on a glass plate but separated from the glass plate by a substrate and/or a coverfilm. Actinic light, such as UV light, is directed against the resin layer through a negative. The result is that the liquid resin is selectively cross-linked and cured to form a printing image surface that mirrors the image on the negative. Upon exposure to actinic radiation, the liquid photopolymer resin polymerizes and changes from a liquid state to a solid state to form the raised relief image. After the process is complete, non-crosslinked liquid resin can be recovered (i.e., reclaimed) from the printing plates and recycled in the process to make additional plates.
Various processes have been developed for producing printing plates from liquid photopolymer resins as described, for example, in U.S. Pat. No. 5,213,949 to Kojima et al., U.S. Pat. No. 5,813,342 to Strong et al., U.S. Pat. Pub. No. 2008/0107908 to Long et al., and in U.S. Pat. No. 3,597,080 to Gush, the subject matter of each of which is herein incorporated by reference in its entirety.
Typical steps in the liquid platemaking process include:
(1) casting and exposure;
(2) reclamation;
(3) washout;
(4) post exposure;
(5) drying; and
(6) detackification.
In the casting and exposure step, a photographic negative is placed on a glass platen and a coverfilm is placed on the negative in an exposure unit. All of the air is then removed by vacuum so that any wrinkling of the negative or coverfilm can be eliminated. Thereafter, a layer of liquid photopolymer and a backing sheet (i.e., a thin layer of polyester or polyethylene terephthalate) are applied on top of the coverfilm and negative. The backing sheet may be coated on one side to bond with the liquid photopolymer and to serve as the back of the plate after exposure. Then upper and/or lower sources of actinic radiation (i.e., UV lights) are used to expose the photopolymer to actinic radiation to crosslink and cure the liquid photopolymer layer in the areas not covered by the negative. The top sources of actinic radiation are used to create the floor layer of the printing plate (i.e., back exposure) while the bottom sources of actinic radiation are used to face expose the photopolymer to actinic radiation through the negative to create the relief image.
After the exposure is complete, the printing plate is removed from the exposure unit and the photopolymer that was not exposed to actinic radiation (i.e., the photopolymer covered by the negative) is reclaimed for further use. In liquid platemaking, resin recovery is an important factor relating to the production of photopolymerizable resin printing plates because the resins used to produce the plates are relatively expensive. In all areas not exposed to UV radiation, the resin remains liquid after exposure and can then be reclaimed. In a typical process, the uncured resin is physically removed from the plate in a process step so that the uncured resin can be reused in making additional plates. This “reclamation” step typically involves squeegeeing, vacuuming or otherwise removing liquid photopolymer remaining on the surface of the printing plate.
Stereolithography is yet another conventional process for providing flexographic printing plates, which is an additive layering process that is very time consuming. Each layer of photopolymer is cured, lifted, back filled with more resin, cured again, and the process is repeated over and over until the required thickness and plate properties are achieved. The process of creating flexographic printing plates using stereolithography can take anywhere from hours to more than a day.
Thus, it would be desirable to provide an improved method of making flexographic printing plates, which involves fewer process steps, is less time consuming, creates less waste, and reliably provides printing plates comprising printing dots with desirable characteristics.